Multilayered gel electrolyte bonded rechargeable electrochemical cell

ABSTRACT

An electrochemical cell 10 includes first and second electrodes 12 and 14 with an electrolyte system 26 disposed therebetween. The electrolyte system includes at least a multilayered first polymeric region 28, having second layers 30 and 32, of a second polymer material. The second layers may absorb an electrolyte active species and to adhere the adjacent layer of electrode material to the electrolyte 26. The electrolyte system further includes a process for packaging and curing the electrolyte after it has been incorporated into a discrete battery device.

This is a Divisional application under §1.60 of U.S. patent applicationSer. No. 08/835,894 filed Apr. 14, 1997 and assigned to Motorola, Inc.now U.S. Pat. No. 5,716,421

TECHNICAL FIELD

This invention relates in general to the field of electrolytes forelectrochemical cells, and more particularly to methods of makingelectrochemical cells using polymer gel electrolytes.

BACKGROUND OF THE INVENTION

There has been a great deal of interest in developing better and moreefficient methods for storing energy for applications such as cellularcommunication, satellites, portable computers, and electric vehicles toname but a few. Accordingly, there has been recent concerted efforts todevelop high energy, cost effective batteries having improvedperformance characteristics.

Rechargeable or secondary cells are more desirable than primary(non-rechargeable) cells since the associated chemical reactions whichtake place at the positive and negative electrodes of the battery arereversible. Electrodes for secondary cells are capable of beingrecharged by the application of an electrical charge thereto. Numerousadvanced electrode systems have been developed for storing electricalcharge. Concurrently much effort has been dedicated to the developmentof electrolytes capable of enhancing the capabilities and performance ofelectrochemical cells.

Heretofore, electrolytes have been either liquid electrolytes as arefound in conventional wet cell batteries, or solid films as areavailable in newer, more advanced battery systems. Each of these systemshave inherent limitations and related deficiencies which make themunsuitable for various applications. Liquid electrolytes, whiledemonstrating acceptable ionic conductivity tend to leak out of thecells into which they are sealed. While better manufacturing techniqueshave lessened the occurrence of leakage, cells still do leak potentiallydangerous liquid electrolytes from time to time. Moreover, any leakagein the cell lessens the amount of electrolyte available in the cell,thus reducing the effectiveness of the device.

Solid electrolytes are free from problems of leakage, however, they havetraditionally offered inferior properties as compared to liquidelectrolytes. This is due to the fact that ionic conductivities forsolid electrolytes are often one to two orders of magnitude poorer thana liquid electrolyte. Good ionic conductivity is necessary to insure abattery system capable of delivering usable amounts of power for a givenapplication. Most solid electrolytes have not heretofore been adequatefor many high performance battery systems.

One class of solid electrolytes, specifically gel electrolytes, haveshown great promise for high performance battery systems. Gelelectrolytes contain a significant fraction of solvents and/orplasticizers in addition to the salt and polymer of the electrolyteitself. One processing route that can be used to assemble a battery witha gel electrolyte is to leave the electrolyte salt and solvent out ofthe polymer gel system until after the cell is completely fabricated.The electrodes and a separator are bonded together in an environmentdevoid of the electrolyte salt. Thereafter, the solvent and theelectrolyte salt may be introduced into the system in order to activatethe battery. While this approach (which is described in, for example,U.S. Pat. No. 5,456,000 issued Oct. 10, 1995) has the advantage ofallowing the cell to be fabricated in a non-dry environment (theelectrolyte salt in a lithium cell is typically highly hygroscopic) itoffers problems with respect to performance and assembly. First, the gelelectrolyte may lack sufficient mechanical integrity to prevent shortingbetween the electrodes while they are being bonded or laminated togetherwith the electrolyte. The electrolyte layer thickness is reported to be75 μm, presumably to overcome this shorting problem and to helpfacilitate handling of the electrolyte material. When compared to the 25μm typical thickness for separators used in liquid lithium ion cells,this results in a significant reduction in the volumetric energy densityfor the cell.

Second, in order to create porosity in the polymer and electrode layersthat will be used to absorb liquid electrolyte, a plasticizer is used.Unfortunately, the subsequent removal of this plasticizer to create thepores requires the use of flammable organic solvents. In addition to thesafety hazard that is created, the time required for the solventextraction process renders it relatively expensive. These problems aresignificant limitations to the successful implementation of gelelectrolytes in electrochemical cells.

An additional issue facing lithium cells, whether fabricated by theprocess described above or not, is that of overheating or self heatingwhich occurs upon, for example, exposure to excessive overvoltage duringcharging. For example, lithiated cobalt oxide materials used as thecathode of lithium ion electrochemical cells will begin to self heatafter being exposed to overvoltage conditions sufficient to raise theinternal cell temperature above about 180 degrees Centigrade. Once thisthreshold has been reached, the cell will continue to heat, degradingcell performance, and potentially posing a burn risk to the user of adevice into which the cell is incorporated

Accordingly, there exists a need for a new electrolyte system whichcombines the properties of good mechanical integrity, as well as theability to absorb sufficient amounts of an electrolyte active species soas to produce an electrolyte system with the high ionic conductivitycharacteristic of liquid electrolytes. The electrolytes so formed shouldalso avoid excessive swelling and all of the problems associatedtherewith. The electrolyte should also have the ability to shut a cellinto which it is incorporated off, once a threshold temperature isreached, thus obviating the problems described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side view of an electrochemical cell inaccordance with the invention;

FIG. 2 is a cross-sectional side view of an electrolyte layer for usewith an electrochemical cell, in accordance with the invention;

FIG. 3 is cycling data for electrochemical cells fabricated inaccordance with the invention;

FIG. 4 is a chart illustrating thermal and voltage behavior of anelectrochemical cell incorporating an electrolyte layer, in accordancewith the invention; and

FIG. 5 is a temperature vs. impedence profile for an electrochemicalcell incorporating an electrolyte layer, in accordance with theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the specification concludes with claims defining the features ofthe invention that are regarded as novel, it is believed that theinvention will be better understood from a consideration of thefollowing description in conjunction with the drawing FIGS, in whichlike reference numerals are carried forward.

Referring now to FIG. 1, there is illustrated therein a cross sectionalside view of a gel electrolyte bonded electrochemical cell in accordancewith the instant invention. The cell 10 includes first and secondelectrodes 12 and 14 respectively. The first electrode may be, forexample, an anode in a lithium rechargeable cell. Accordingly, the anodemay be fabricated of any of a number of different known materials forlithium rechargeable cells, examples of which include metallic lithium,lithium alloys, such as lithium: aluminum, and lithium intercalationmaterials such as carbon, petroleum coke, activated carbon, graphite,and other forms of carbon known in the art. In one preferred embodiment,the anode 12 is fabricated of an amorphous carbonaceous material such asthat disclosed in commonly assigned, co-pending U.S. pat. applicationSer. No. 08/561,641 entitled "Improved Carbon Electrode Materials ForLithium Battery Cells And Method of Making Same" filed on Nov. 22, 1995,in the names of Jinshan Zhang, et al., the disclosure of which isincorporated herein by reference.

More particularly, the anode 12 comprises a layer of active material 16such as a carbon material as described hereinabove deposited on asubstrate 18. Substrate 18 may be any of a number of materials known inthe art, examples of which include copper, gold, nickel, copper alloys,copper plated materials, and combinations thereof. In the embodiment ofFIG. 1, the substrate 18 is fabricated of copper. The second electrode14 may be adapted to be the cathode of a lithium rechargeable cell. Insuch an instance, the cathode is fabricated of the lithium intercalationmaterial such as is known in the art, examples of which includelithiated magnesium oxide, lithiated cobalt oxide, lithiated nickeloxide, and combinations thereof. In one preferred embodiment, thecathode 14 is fabricated of a lithiated nickel oxide material such as isdisclosed in commonly assigned, co-pending U.S. patent application Ser.No. 08/464,440 in the name of Zhenhua Mao filed Jun. 5, 1995, thedisclosure of which is incorporated herein by reference.

More particularly, the cathode 14 comprises a layer of the cathodeactive material 20 disposed on a cathode substrate 22. The cathodematerial 20 maybe such as that described hereinabove, while thesubstrate may be fabricated from any of a number of known materialsknown in the art, examples of which include aluminum, nickel, andcombinations thereof. In one preferred embodiment, substrate 22 isfabricated of aluminum.

Disposed between electrodes 12 and 14 is a layer of an electrolytematerial system 26. The electrolyte system 26 comprises an electrolyteactive species and a polymer gel electrolyte support structureconsisting of at least two different polymers. A first polymer isprovided as an absorbing phase and the second polymer is provided as aninert phase. The inert phase 28 is provided to give mechanical integrityand structural rigidity to the electrolyte system. The absorbing phase30, 32, which may be disposed on either or both sides of the inertphase, is adapted to engage the electrolyte active species therein. Thegelling polymer may further act as a bonding paste to assist in adheringthe electrodes to the inert polymer.

The electrolyte active species is a liquid or solid component (or both)which provides ionic conductivity between the anode and the cathode. Inthe embodiment in which the electrochemical cell 10 is a lithiumintercalation cell, the electrolyte active species consists of an alkalimetal salt in a solvent. Typical alkali metal salts include, but are notlimited to, salts having the formula M⁺ X⁻ where M⁺ is an alkali metalcation such as Li⁺, Na⁺, K⁺, and combinations thereof; and X⁻ is ananion such as. Cl⁻, Br⁻, I⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, ASF₆ ⁻, SbF₆ ⁻, CH₃CO₂ ⁻, CF₃ SO₃ ⁻, N(CF₃ SO₂)₂ ⁻, C(CF₃ SO₂)₃ ⁻ and combinations thereof.The solvent into which the salt is dispersed is typically an organicsolvent including, but not limited to, propylene carbonate (PC),ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate(DMC), dipropylcarbonate, dimethylsulfoxide, acetonitrile,dimethoxyethane, tetrahydrofuran, n-methyl-2-pyrrolidone (NMP), acetoneand combinations thereof. For other electrode combinations, i.e., Ni--Cdor Ni-metal hydride, other electrolyte active species may be used, suchas KOH.

Referring now to FIG. 2, there is illustrated therein a cross-sectionalside view of the electrolyte system 26 of FIG. 1. The electrolyte system26 comprises a first polymer region 52 is a porous separator which isformed of one or more layers of inert polymer material. The term inertrefers to the fact that the material itself is not absorbing, though thelayer of material, due to its porosity (as described below) may beabsorbing. In the preferred embodiment of the instant invention, thefirst polymer region 52 is actually a multilayered polymer regioncomprising two or more different polymers. In the embodiment of FIG. 2,the first region comprises three polymer layers, a first layer of afirst polymer 54 sandwiched between two layers 56 and 58 of a secondpolymer. Alternatively layers 56 and 58 may be fabricated of differentpolymer materials.

The polymers from which layers 54, 56, and 58 may be fabricated areselected from the group of materials consisting of polyalkenes such aspolyethylene, polypropylene, or other polymers such aspolytetrafluroethylene, polyethyleneterephthalate, polystyrene, ethylenepropylene diene monomer, nylon, and combinations thereof. In onepreferred embodiment, layer 54 is fabricated of polyethylene, whilelayers 56 and 58 are fabricated of polypropylene.

First region 52 includes first and second major surfaces 60 and 62,respectively, and wherein disposed on at least one of the first andsecond major surfaces is a layer of an absorbing or gel-forming polymer70. The absorbing or gel-forming polymer may be selected from the groupof polymers, including polyvinylidene fluoride (PVDF), polyurethane,polyethylene oxide, polyacrylonitrile, polymethylacrylate,polyacrylamide, polyvinylacetate, polyvinylpyrrolidone,polytetraethylene glycol diacrylate, copolymers of any of the foregoing,and combinations thereof. As illustrated in FIG. 2, the layer of thesecond polymeric material 70 is disposed on surface 60 of region 52. Asecond layer 72 of a second polymeric material may be disposed on thesecond major surface 62 of region 52. The layers of gel forming polymer70, 72 may be fabricated of the same or different materials, asdescribed hereinabove.

An advantage of the instant system is that the layers of the firstregion 52, when heated above a threshold temperature, for example about135 degrees Centigrade (° C.) for poly ethylene, the material melts andfuses essentially choking off all ionic conductivity. Thus, anycontinued application of, for example a overheating overcharge, willresult in a fused polymer layer. This means an end of the applied chargevoltage since the fused polymer cuts off all ionic conductivity betweenthe electrodes. Thus, the heating ends, and a potentially undesirablesituation is avoided. The benefit of providing the sandwiching layersof, for example, polypropylene, is to assure a continued electrical andphysical barrier between the electrodes, since any contact would resultin a short, and potentially the type of heating which is to be avoided.Poly propylene remains relatively intact up to about 165° C.

Returning to the layers of absorbing or gel forming polymer, such layersmay alternatively be deposited on the electrodes, and subsequently putinto contact with the first polymer region when the electrodes and theinert polymer are stacked together to complete the battery cell. Moreparticularly, a layer of the absorbing polymer may be coated onto atleast one surface 15 (of FIG. 1) of, for example, the cathode. Surface15 is ultimately disposed adjacent the electrolyte system 26: Hence,when the first polymer region is arranged in stacked configuration withthe electrodes, the absorbing polymer is disposed in contact with thefirst polymer region.

The electrodes and separator materials described above may be fabricatedinto electrochemical cells by winding and/or stacking the layers ofelectrode and separator material. Discrete cells are then packagedbetween sheets of a vapor impermeable package as is illustrated in FIG.3. More particularly, discrete cell 90, is packaged between sheets ofwater vapor impermeable material such as metal foil laminates. Sheets 92and 94 enclose the discrete package, or cell. Either before or after thepackage is sealed, the electrolyte active material, as described above,is injected into it.

The battery cell is then cured by exposing it both to a compression andheating step. More particularly, the packaged discrete battery cell isexposed to a temperature of between 50° and 150° C. for a period of timebetween 6 and 3600 seconds. The exact time will depend on the size ofthe cells themselves. The compression force used to bond and cure andbattery pack is on the order of between 1 and 500 lbs/cm² and preferablybetween 50 and 100 lbs/cm². This heating and pressing step results inthe absorbing polymer material being dissolved, along with the liquidelectrolyte active species, wherein they seep or are forced into thepores of the inert polymer. When the cell cools and solidifies, or"gels" it serves the additional function of adhering the layers ofelectrode material to the separator.

The invention may be better understood from a perusal of the examples ofwhich are attached hereto.

EXAMPLES Example 1

Commercially available polypropylene, known as Celgard^(R) 2300 wascoated with PVDF by a dip coating process using an MEK/2-butanolsolution. Three flat lithium ion polymer cells were built using the PVDFcoated poly propylene and a standard electrolyte 60:40 EC/DEC/1MLiPF6!). The anode was fabricated of a commercially available graphitematerial, while the cathode was fabricated of a commercially availableLiCoO2 material. The cells were cycled initially through 5 cycles(formation) and then through a further 20 cycles. The performance ofthese cells in general was similar to the standard cells. The cyclingbehavior is shown in FIG. 3. Two of these cells were then subjected toovercharge (@3C rate) safety tests. Performance was consistent with whatwould be expected for other cells.

The two cells were charged to 4.2 V at C/2 charge rate. The cells werethen overcharged at 1.5 A (>3C) to a 10 V limit. Results are shown inFIG. 4. The cell skin temperature and cell voltage increased until thevoltage was driven to the power supply limit (10 V). The current thendropped to a negligible value and the temperature returned to ambient.The low current confirms a large increase in cell impedance.

To confirm the "shutdown" properties of the two separators, the internalcell impedance vs. temperature was measured. (These tests were run ondischarged cells for safety reasons, however, impedance differencesbetween charged/discharged state are relatively small.) Cells wereheated at 1° C./minute in a Thermotron from 25° to 175° C., followed bya "soak" at 175° C. for 10 minutes and then cooling to 25° C. @ 1° C./min. Results for the trilayer and the conventional polypropylenematerials are shown in FIG. 5. Two important conclusions can be madefrom FIG. 5:

1) "Shutdown" in the trilayer occurs at a much lower temperature (i.e.,135° C.); and

2) The impedance increase in the trilayer is substantially greater thanin the polypropylene material. Since this large impedance will preventvirtually all current flow in the cell, the trilayer is expected toprovide a much more effective shutdown mechanism than the polypropylenematerial alone.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as defined by theappended claims.

What is claimed is:
 1. An electrolyte system for a rechargeableelectrochemical cell, said system comprising:a porous separator elementhaving at least a first and a second major surfaces, and fabricated as amultilayered polymer region, wherein the polymers of said multilayeredregion are selected from the group consisting of polyethylene,polypropylene, polytetrafluroethylene, polystyrene,polyethyleneterephthalate, ethylene propylene diene monomer, nylon, andcombinations thereof; a gelling polymer disposed on at least one majorsurface thereof, said gelling polymer selected from the group consistingof polyvinylidene fluoride (PVDF), polyurethane, polyethylene oxide,polyacrylonitrile, polymethylacrylate, polyacrylamide, polyvinylacetate,polyvinylpyrrolidone, polytetraethylene glycol diacrylate, copolymers ofany of the foregoing, and combinations thereof; and an electrolyteactive species dispersed in at least said gelling polymer.
 2. Anelectrolyte system as in claim 1, wherein said electrolyte activespecies comprises an electrolyte salt dispersed in an organic solvent.3. An electrolyte system as in claim 2, wherein said organic solvent isselected from the group consisting of propylene carbonate, ethylenecarbonate, diethyl carbonate, dimethyl carbonate, dipropylcarbonate,dimethylsulfoxide, acetonitrile, dimethoxyethane, tetrahydrofuran,n-methyl-2-pyrrolidone, and combinations thereof.
 4. An electrolytesystem as in claim 2, wherein said electrolyte salt is selected from thegroup consisting of Cl⁻, Br⁻, I⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, ASF₆ ⁻, SbF₆ ⁻,CH₃ CO₂ ⁻, CF₃ SO₃ ⁻, N(CF₃ SO₂)₂ ⁻, C(CF₃ SO₂)₂ ⁻ and combinationsthereof.
 5. An electrolyte system as in claim 1, wherein saidmultilayered polymer region is formed of three layers.
 6. An electrolytesystem as in claim 5, wherein said three layers include a first polymerlayer formed of poly ethylene disposed between layers of a secondpolymer.
 7. An electrolyte system as in claim 6, wherein said layers ofsaid second polymer are fabricated of poly propylene.
 8. Anelectrochemical cell comprising:an anode; a cathode; and an electrolytesystem comprising a porous separator element having at least a first anda second major surfaces, and fabricated as a multilayered polymerregion, wherein the polymers of said multilayered region are selectedfrom the group consisting of polyethylene, polypropylene,polytetrafluroethylene, polystyrene, polyethyleneterephthalate, ethylenepropylene diene monomer, nylon, and combinations thereof, and furtherincluding a gelling polymer disposed on at least one major surfacethereof, said gelling polymer selected from the group consisting ofpolyvinylidene fluoride (PVDF), polyurethane, polyethylene oxide,polyacrylonitrile, polymethylacrylate, polyacrylamide, polyvinylacetate,polyvinylpyrrolidone, polytetraethylene glycol diacrylate, copolymers ofany of the foregoing, and combinations thereof and an electrolyte activespecies dispersed in at least said gelling polymer.
 9. Anelectrochemical cell as in claim 8, wherein said electrolyte activespecies comprises an electrolyte salt dispersed in an organic solvent.10. An electrochemical cell as in claim 9, wherein said organic solventis selected from the group consisting of propylene carbonate, ethylenecarbonate, diethyl carbonate, dimethyl carbonate, dipropylcarbonate,dimethylsulfoxide, acetonitrile, dimethoxyethane, tetrahydrofuran,n-methyl-2-pyrrolidone, and combinations thereof.
 11. An electrochemicalcell as in claim 9, wherein said electrolyte salt is selected from thegroup consisting of Cl⁻, Br⁻, I⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, ASF₆ ⁻, SbF₆ ⁻,CH₃ CO₂ ⁻, CF₃ SO₃ ⁻, N(CF₃ SO₂)₂ ⁻, C(CF₃ SO₂)₂ ⁻ and combinationsthereof.
 12. An electrochemical cell as in claim 8, wherein saidmultilayered polymer region is formed of three layers.
 13. Anelectrochemical cell as in claim 8, wherein said gelling polymer isPVDF.
 14. An electrochemical cell as in claim 12, wherein saidmultilayered polymer comprises a first polymer layer formed of polyethylene disposed between layers of a second polymer.
 15. Anelectrochemical cell as in claim 13, wherein said layers of said secondpolymer are fabricated of poly propylene.